In web printing and drying operations, it is often desirable that the web be contactlessly supported, in order to avoid damage to the web itself or to the coating (such as ink) previously applied to one or more surfaces of the web. One conventional arrangement for contactlessly supporting a web includes horizontal upper and lower sets of air bars between which the web travels. Hot air issuing from the air bars both dries and supports the web. Occasionally it becomes necessary to change the direction of web travel while maintaining the contactless environment. This can be accomplished using air turns, which are devices that support a flexible web on a cushion of air pressure as the web travels around a curved path. Air turns such as the Tecturn.RTM., commercially available from W. R. Grace & Co.-Conn., accomplishes web guidance without regard to any web drying function.
GB 2 126 974B (the disclosure of which is hereby incorporated by reference) discloses a device for contactlessly supporting a moving web by guiding the same around a turn, whereby a substantial amount of web drying is also accomplished. To that end, the device disclosed includes a plurality of elongate air bars, each having opposite side walls, and a front wall that is located between the side walls and has opposite longitudinally extending edge portions which are spaced from the side walls to cooperate with them in defining a pair of air outlet slots, each extending along the length of the air bar, the edge portions of the front wall being curved widthwise towards the middle of the air bar so that each of the outlet slots comprises a Coanda type nozzle for directing pressurized air forwardly from the interior of the air bar and laterally across the from wall toward the other outlet slot. A plenum chamber communicates with a source of pressurized air, and is also in communication with the inside of each air bar. The air bars are arranged such that they lie substantially in a convex arc. Thus, such devices or combine the features of an air turn and a flotation dryer. The air bars making up the air turn support the web without contact on a cushion of air while the web follows a circular path and is simultaneously heated and dried. Air turns of this type are often combined with straight path flotation systems, or a plurality of such air turns can be used to create an "S" shaped web path. As shown in U.S. Pat. No. 4,218,833 (the disclosure of which is hereby incorporated by reference), such air turns can be arranged with adjacent arrays inverted in relation to each other so as to support the traveling web in a circuitous path around such arrays.
An important aspect of any flotation system is the stability of the web as it passes over an air bar. Airflow instabilities near the web can induce web flutter and subsequent web contact with mechanical parts of the drying, resulting in coating disturbance or web damage. Web flutter can be manifested in a multitude of forms, ranging from a violent flapping of the web to a high frequency drumming.
Excessive web flutter has been encountered in air turn applications. Where a plurality of air turns are used together so that the web follows a sinusoidal path, web flutter has been encountered as the web leaves the lower air turn and before it reaches the upper air turn. It is believed that such web flutter may be due to distance between turns; the spent air from the lower turn influences the web to follow the turn diameter, while the upper turn pushes the web out to maintain web flotation. In particular, it is believed that the cause of the fluttering is related to the angle of the lower turn being greater than 90.degree.. The exhausted air from the turn face apparently exits upward with the web sheet and is peeling off away from the bottom of the web sheet face, causing vortices.
It is therefore an object of the present invention to minimize or eliminate web flutter in air turn applications.